Evidence from benthic foraminifera and stable carbon isotopes
(Paläo-) Produktivität im Holozän und Letzten Glazialen Maximum vor Marokko aus benthischen Foraminiferen und stabilen Kohlenstoffisotopen
Astrid Eberwein
Ber. Polarforsch. Meeresforsch. xxx (2006)
ISSN 1618-3193
Alfred Wegener Institute for Polar and Marine Research Am Alten Hafen 26
27568 Bremerhaven Germany
e-mail: aeberwein@awi-bremerhaven.de
Die vorliegende Arbeit ist die inhaltlich unveränderte Fassung einer kumulativen Dissertation, die im Juni 2006 dem Fachbereich Geowissenschaften der Universität Bremen vorgelegt wurde.
Page Table of contents... I-II Abstract... III Zusammenfassung... IV Danksagung... V
1. INTRODUCTION... 1
1.1 Investigation area... 1
1.1.2 Present-day conditions... 2
1.1.3 Past conditions... 3
1.2 Brief overview of benthic foraminiferal research... 4
1.3 Aim of this thesis... 6
1.3.1 Objectives... 6
1.3.2 Overview of Research... 7
2. PUBLICATIONS... 8
2.1 Manuscript 1... 8
Regional primary productivity differences off Morocco (NW-Africa) recorded by modern benthic foraminifera and their stable carbon isotopic composition 2.2 Manuscript 2... 38
The influence of organic matter fluxes on the microhabitat distribution of live benthic foraminifera off Cape Ghir and Cape Yubi (NW-Africa) 2.3 Manuscript 3... 56
Last Glacial Maximum paleoproductivity and water masses off NW-Africa: evidence from benthic foraminifera and stable isotopes 3. CONCLUSIONS & FUTURE PERSPECTIVES... 77
3.1 Modern benthic foraminifera and their stable carbon isotopic composition reflect primary productivity... 77
3.2 Dependance of benthic foraminiferal microhabitats on organic matter fluxes... 77
3.3 Reconstruction of paleoproductivity and water masses during the LGM... 78
3.4 Future perspectives... 79
APPENDIX... 93
Taxonomic list... 93
Plate I... 100
Plate II... 102
Abstract
I used the distribution and composition of benthic foraminiferal faunas as well as the stable carbon isotopic signals of benthic foraminiferal tests as proxy in determining and reconstructing (paleo-) productivity during the Holocene and the Last Glacial Maximum (LGM). The investigation area is located between Cape Yubi (27.5°N) and Cape Ghir (31°N) off Morocco (NW-Africa). Present chloropyll-a distributions in surface waters inferred from satellite imagery indicate that the investigated stations cover different productivity regimes. First, benthic foraminifera were calibrated as proxy regarding productivity. In this context, the influence of different environmental parameters on live (Rose Bengal stained) and dead species in sediment surface samples (0 – 1 cm) was examined. Furthermore, the impact of organic carbon fluxes on the microhabitat distribution of live benthic foraminifera (0 – 10 cm) was investigated. Finally, the potential of benthic foraminifera as productivity proxy was used to reconstruct paleoproductivity during the LGM.
The results revealed a high productivity variability in the upwelling region off Morocco in the Holocene and during the LGM. Generally, the local productivity during the LGM was more heterogeneous than presently. It turned out that productivity, and the resulting organic matter export to the sea-floor, are the most important factors controlling the horizontal as well as vertical distribution and composition of Holocene benthic foraminiferal faunas. In the cape regions, identical live and dead assemblages, high standing stocks, and low species δ13C values reflect the highly productive Cape Ghir and Cape Yubi filaments. The faunal succession from the shelf to the deep sea, the decrease in total standing stocks and the increase of the average living depth of Globobulimina affinis reflect the decrease in organic matter fluxes. By contrast, the area between the capes is characterised by differently composed live and dead faunas, low standing stocks, and higher δ13C values, thus reflecting low to slightly seasonally varying productivity in the Holocene.
During the LGM, paleoproductivity was higher in the area between the capes than in the Holocene, whereas paleoproductivity was comparably high off the capes. Four major regions with characteristic productivity regimes were distinguished: (1) the region off Cape Yubi was characterised by highest paleoproductivity; (2) west and southwest off Cape Ghir moderate to high, seasonally fluctuating paleoproductivity was predominant; (3) the region southwest off Cape Yubi was characterised by moderate, highly seasonally varying paleoproductivity; and (4) the region further offshore Cape Ghir was characterised by highest seasonally varying paleoproductivity.
Zusammenfassung
Im Rahmen dieser Studie diente die faunistische Zusammensetzung und Verbreitung benthischer Foraminiferen, sowie die stabilen Kohlenstoff-Isotopenverhältnisse ihrer Gehäuse als Indikatoren zur Erkennung der Primärproduktion im Holozän und zur Rekonstruktion der Paläoproduktion während des Letzten Glazialen Maximums (LGM). Das Arbeitsgebiet befindet sich zwischen Kap Ghir (31°N) und Kap Yubi (27.5°N) vor der marokkanischen Küste (NW-Afrika). Rezente Chlorophyll-a Gehalte im Oberflächenwasser verdeutlichen, dass die analysierten Proben unterschiedliche Produktivitätsregime abdecken. Benthische Foraminiferen wurden zunächst bezüglich der Produktivität geeicht, um sie anschliessend als leistungsfähigen Paläoproduktivitätsproxy für das LGM einzusetzen. Für diese Kalibrierung wurde der Einfluss unterschiedlicher Umweltparameter auf die Zusammensetzung und Verteilung der Lebend- und Totfauna und die Auswirkung von organischen Kohlenstoffflüssen auf die Mikrohabitatverteilung lebender benthischer Foraminiferen des Holozäns untersucht.
Die Ergebnisse bestätigen, dass das Auftriebsgebiet vor Marokko im Holozän durch starke Pro- duktivitätsunterschiede gekennzeichnet ist und verdeutlichen, dass die regionale Produktivitätsvariabili- tät im LGM deutlich stärker ausgeprägt war. Es wurde gezeigt, dass der entscheidendste Faktor, der die holozäne horizontale und vertikale Verteilung und Zusammensetzung benthischer Foraminiferenfaunen bestimmt, die Primärproduktion und der Export von organischer Substanz zum Meeresboden ist. In den Kapregionen wird die hohe Produktivität der Kap Ghir und Kap Yubi Filamente durch identische Lebend- und Totfaunen, hohe Siedlungsdichten sowie niedrige δ13C Werte reflektiert. Die Abfolge der benthischen Foraminiferenvergesellschaftungen entlang des Transekts vom Schelf zur Tiefsee, die Abnahme der Siedlungsdichten und die Zunahme der mittleren Lebenstiefe von Globobulimina affinis zeigen den abnehmenden Export organischen Materials an. Im Gegensatz dazu ist die Region zwischen den beiden Kaps durch niedrige bis saisonal schwankende Primärproduktion charakterisiert. Dies spiegelt sich in unterschiedlichen Lebend- und Totfaunen, niedrigen Siedlungsdichten sowie höheren δ13C Werten wider.
Im LGM war die Paläoproduktivität im Gebiet zwischen den Kaps erhöht im Vergleich zum Holozän, wohingegen sie vor den Kaps vergleichbar war. Im LGM lassen sich vier Regionen mit cha- rakteristischen Paläoproduktivitätsregime unterscheiden: (1) Die Region vor Kap Yubi zeichnete sich durch höchste Paläoproduktivität aus; (2) in der Region vor Kap Ghir war moderate bis hohe, saisonal schwankende Paläoproduktivität vorherrschend; (3) die Region südwestlich von Kap Yubi war durch moderate, saisonal stark variierende Paläoproduktivität charakterisiert; und (4) in der Region vor Kap Ghir existierte die am stärksten saisonal variierende Paläoproduktivität.
Danksagung
Mein erster Dank gilt meinem Doktorvater Prof. Dr. Andreas Mackensen für die Vergabe und Betreuung dieser Arbeit. Ich danke Dir für die enthusiastische Diskussionsbereitschaft und konstruktive Kritik, die mich immer wieder motiviert haben, sowie die Unterstützung bei der Teilnahme an Expeditionen und Konferenzen. Vielen herzlichen Dank für Deine Unterstützung und Prioritätensetzung bei der Umsetzung meiner Pläne in der Endphase! Herrn Prof. Dr. Gerold Wefer danke ich für die Übernahme des Zweitgutachtens.
Ich danke der Besatzung des Forschungsschiffes Meteor für ihre ausgezeichnete Arbeit während der Expeditionen, die erst die Bearbeitung des Probenmaterials im Rahmen dieser Dissertation ermöglicht hat, sowie die angenehme Atmosphäre.
Für die exzellente technische Unterstützung, vielfältige und geduldige Hilfe im Labor und am REM, das Suchen und Finden diverser Proben danke ich Ute Bock, Volker Diekamp, Beate Hollmann, Gerd Kuhn, Almuth Maschner, Günther Meyer, Birgit Meyer-Schack, Maike Scholz, Monika Segl, Susanne Wiebe und Alexius Wülbers.
Viele Kollegen haben durch ihr Interesse, ihre Diskussionsbereitschaft und das Bereitstellen von Rohdaten zum erfolgreichen Gelingen dieser Arbeit beigetragen. Ich danke Sylvia Brückner, Catalina Gebhardt, Helena Filipsson, Tim Freudenthal, Peer Helmke, David Heslop, Claus-Dieter Hillenbrand, Ulrike Holzwarth, Sabine Kasten, Holger Kuhlmann, Laetitia Licari, Helge Meggers, Susanne Neuer, Gerhard Schmiedl, Stefanie Schumacher, Dorit Siggelkow, Silke Steph, Julia Thiele und Michelle Zarrieß.
Meinen Freunden einen ganz lieben und herzlichen Dank für die vielen schönen und lustigen Freizeitaktivitäten und die Sonnennachmittage und -abende am Deich, insbesondere in der Endphase der Promotion, die mir immer wieder positive Energie gegeben haben und mir gezeigt haben, dass DAS LEBEN LACHT! Ich habe wieder mehr Zeit für‘s Powershoppen und hoffe, Ihr kommt alle mit nach Dubai :-) Meiner Familie gilt ein ganz spezieller Dank für die Unterstützung in vielerlei Hinsicht.
‘Jedem Anfang wohnt ein Zauber inne‘ (Hermann Hesse).
1. INTRODUCTION
Ocean margins are high-production systems and, therefore, provide high resolution sedimentary archives, in which significant information of Earth’s climate history is stored. Coastal upwelling areas are of particular interest, since the biological production accounts for 80 % of the total marine new production (Berger et al., 1989). This marine production is affected by the global and, particularly, regional atmospheric and oceanographic circulation. Enhanced atmospheric circulation is suggested to be responsible for stronger upwelling, which in turn increases the primary productivity in surface waters. Amongst other factors, organic matter production is considered to control the exchange of the greenhouse gas carbon dioxide between the atmosphere and ocean, which in turn causes present and past climate variations.
Benthic foraminifera are a valuable proxy for paleoceanographic reconstructions, since they have a long geological record, an ubiquitous distribution and their calcareous tests have a high fossilisation potential. Distinct benthic foraminiferal faunal compositions and the stable isotopic composition of their calcareous tests in Recent sediments provide substantial information about the association with present environmental conditions. In this context, the attention is focused on benthic foraminifera as a useful proxy for the export production to the sea floor, bottom and pore water oxygen concentrations and bottom water mass circulation.
It is important to increase our knowledge about the influence of productivity on the distribution and composition of modern benthic foraminiferal faunas and the stable isotopic composition of their tests, since this knowledge serves as basis for a detailed reconstruction of paleoproductivity, which allows conclusions about past climate changes. In this context, it is essential to conduct spatially highly resolved investigations to detect local heterogeneities and to reliably reconstruct paleoproductivity.
Therefore, I focus on the characterisation of modern benthic foraminifera and their preferences regarding productivity and its influence on the stable carbon isotopic composition of their tests in a regionally restriced area within the upwelling region off Morocco (NW-Africa). Subsequently, benthic foraminifera are used as proxy for a detailed reconstruction of past productivity conditions during the Last Glacial Maximum (LGM).
1.1 Investigation area
The investigation area is located in the upwelling region off Morocco at the NW-African continental margin. Cape Ghir (31°N) and Cape Yubi (27.5°N) restrict this area in the north and south, respectively. The Canary Islands, which are located 100 to 600 km off the African coast at a latitude of 28° - 29°N, form the western boundary. The broad Moroccan continental shelf is characterised by an extension between 30 km and 100 km. A relatively narrow shelf with an extension of 25 km off Cape Ghir and a broader shelf with an extension of 75 km off Cape Yubi was observed (Summerhayes et al., 1976).
Fig. 1: Study area with chlorophyll-a distributions (mean March 1998; Helmke, pers. comm) in surface waters.
Direction of surface currents (= Canary Current) after Mittelstaedt (1991) and NE trade winds are indicated by arrows. Squares mark investigated Holocene and LGM (squares with crosses) sample locations.
1.1.2 Present-day conditions
The study area is part of the large-scale N-Atlantic recirculation system, which combines the Gulf Stream via the Azores Current (AC) and the Canary Current (CC) with the North Equatorial Current. The AC splits into three branches south of the Azores Islands, of which the eastern most branch, the southwards directed CC, has a major influence on the coastal upwelling off Morocco. The flow path of the CC either through the Canary Islands or along the Moroccan coast is seasonally variable (Stramma and Siedler, 1988; Knoll et al., 2002) and it increases during summer and fall (Knoll et al., 2002). The well-mixed surface layer is underlein by the southward directed North Atlantic Central Water (NACW) down to 600 m. Below the NACW the southward directed Mediterranean Outflow Water (MOW), occurs between 600 m - 1700 m and is characterised by high salinity. Between 600 m - 1000 m depth the northward flowing nutrient-enriched Antarctic Intermediate Water was observed close to the coast (Knoll et al., 2002; Llinás et al., 2002). The southward directed nutrient-depleted North Atlantic Deep Water (NADW) occurs between 1700 m and 4000 m and the northward directed nutrient- enriched Antarctic Bottom Water (AABW) below 4000 m (Sarnthein et al., 1982).
The NW-African–Iberian upwelling region, belongs to one of the four major eastern boundary upwelling systems of the world (Hagen, 2001; Carr, 2002) and is characterised by high marine production.
The high primary production is reflected by enhanced chlorophyll-a concentrations in surface waters as inferred from satellite imagery (Fig. 1).
Primary productivity is linked to the interaction of the CC, the coastal morphology and variations in NE trade wind intensity. Coastal upwelling is coupled to the latitudinal northwards shifts of the subtropical high (Azores High) and tropical low pressure areas connected to the migration of the Inter Tropical Convergence Zone. North of 25°N seasonal upwelling is observed with strongest upwelling events in boreal summer and fall (Mittelstaedt, 1991; Van Camp et al., 1991; Nykjaer and van Camp, 1994).
Off Morocco, upwelling of NACW is restricted to a maximum offshore extent of 70 km (Mittelstaedt, 1991; Hernández-Guerra and Nykjaer, 1997; Hagen, 2001). However, particularly, at cape locations the development of filaments is observed, which are characterised by high nutrient concentrations. The filaments can be advected several hundred kilometers into the open ocean (Van Camp et al., 1991;
Nykjaer and van Camp, 1994; Hagen et al., 1996; Hernández-Guerra and Nykjaer, 1997; Parrilla et al., 1999), and thus are an important transport mechanism of nutrients into the oligotrophic open ocean.
1.1.3 Past conditions
During the last glacial / interglacial transition the trade wind belt had almost the identical latitudinal extension as today (Sarnthein et al., 1981; Hooghiemstra et al., 1987; Hooghiemstra, 1989).
However, the trade wind intensity was considered to be enhanced compared to modern conditions as a result of the stronger atmospheric temperature and pressure gradient (Sarnthein et al., 1981; Hooghiemstra et al., 1987; Hooghiemstra, 1989). During the LGM, the sea level was by about 120 m lower than presently (Fairbanks, 1989). The influence of sea level changes on the productivity record has been recently discussed. Martinez et al. (1999) and Bertrand et al. (2000) showed that sea level changes are responsible for zonal shifts of upwelling centers, which in turn control productivity. The assumption that the NW-African upwelling region is the prime example of generally enhanced producitvity during glacial times (Sarnthein et al., 1982) has to be reconsidered. Cape Blanc (21°N), the today’s centre of year-round upwelling activity was characterised by lower productivity during the LGM (Bertrand et al., 1996; Martinez et al., 1999; Zhao et al., 2000; Sicre et al., 2001). In contrast, higher glacial productivity seemed to have been geographically restricted to the northern part of the NW-African upwelling system.
Enhanced productivity was observed at 25°N (Abrantes, 2000; Sicre et al., 2000; Ternois et al., 2000;
Zhao et al., 2000), and in the Canary Islands region (Freudenthal et al., 2002; Henderiks et al., 2002;
Moreno et al., 2002; Kuhlmann et al., 2004). Therefore, the NW-African margin has been described as a region with intense productivity variations of regional significance during glacials (Bertrand et al., 1996; Martinez et al., 1999; Sicre et al., 2001).
1.2 Brief overview of benthic foraminiferal research
Benthic foraminifera (Order FORAMINIFERIDA Eichwald 1830) are eukaryotic, single-celled, and mostly multiple chambered organisms (Sen Gupta, 1999). Presently, about 1000 living genera and approximately 6000 extant species are described (Loeblich and Tappan, 1988), and the first occurence of test fragments was accounted for Cambrian sediments. Benthic foraminifera inhabitat all aquatic environments and are major contributors of the meiofaunal (> 50%) and macrofaunal biomass (Gooday et al., 1992). They are known to have a wide range of different diets (Lipps, 1983; Caralp, 1989a; 1989b;
Murray, 1991; Bernhard and Bowser, 1992; Gooday et al., 1992; Gooday, 1994). Benthic foraminiferal tests are either composed of calcite, constructed from foreign particles or organic material by the foraminifera itself (Gooday, 2001). Particularly, the calcareous specimens have a great fossilisation potential at least above the calcite compensation depth.
The first classification of benthic foraminifera was carried out by d’Orbigny (1826) and was followed by extensive and long-lasting taxonomic work (e.g. Brady, 1879; Brady, 1881; Brady, 1884;
Cushman, 1918-1931; Heron-Allen and Earland, 1922; Cushman, 1948; Phleger and Parker, 1951).
Subsequently, descriptions of the bathymetrical and geographical distribution patterns of benthic foraminifera species were conducted (e.g. Phleger, 1960; Lutze, 1980) and a correlation between benthic foraminifera and physicochemical water mass characteristics was suggested (Streeter, 1973; Schnitker, 1974; Schnitker, 1979; Schnitker, 1980; Murray et al., 1986). The staining method with Rose Bengal invented by Walton (1952) enabled to distinguish between tests containing protoplasm and those being empty.
It was shown that benthic foraminifera live above (truely epibenthic) (Lutze and Thiel, 1989), on (epibenthic) and within (endobenthic) the sediment (Corliss, 1985; Gooday 1986) with clear microhabitat preferences (Mackensen and Douglas, 1989; Rathburn and Corliss, 1994; Mackensen et al., 2000; Schmiedl et al., 2000). This distinct vertical zonation is predominantly influenced by the organic carbon flux and dissolved bottom water oxygen concentrations (Jorissen et al., 1995). In this context some species have been identified to track distinct redox boundaries in the porewater (Corliss and Emerson, 1990; Jorissen et al., 1998; Fontanier et al., 2002; Licari et al., 2003). It is generally accepted that the organic carbon flux is the essential parameter in controlling the benthic foraminifera community (e.g. Corliss and Emerson, 1990; Loubere, 1991), as long as the bottom water or pore water oxygen concentration does not fall below the critical threshold, which has a considerable impact on the benthic foraminiferal occurence and distribution as well (Bernhard, 1992; Jorissen 1998; Loubere, 1994; Kaiho, 1994, 1999; Schönfeld, 2001; Geslin et al., 2004; Sen Gupta and Machain-Castillo, 1993) Quantity and mode of organic matter fluxes control the absence or presence of distinct species (e.g.
Gooday, 1988; Rathburn and Corliss, 1994; Gooday, 1996; Jannink et al., 1998; de Rijk et al., 2000;
Schmiedl et al., 2000; Morigi et al., 2001) as well as the benthic foraminiferal faunal composition and vertical stratification within the sediment (e.g. Jorissen et al., 1992; Mackensen et al., 1995; Loubere,
1997; Jorissen et al., 1998; Fontanier et al., 2002; Gooday and Hughes, 2002; Eberwein et al., subm).
Furthermore, the benthic foraminiferal density is controlled by the organic carbon export (Lutze and Coulbourn, 1984; Altenbach, 1988; Gooday, 1994; Fontanier et al., 2002; Heinz et al., 2002).
The stable carbon isotopic composition of benthic foraminifera tests was a further focus in the researcher’s intererest. In this context, the main emphasis in using these stable carbon isotopic compositions lies on productivity influence (Zahn et al., 1986; Loubere, 1987; Mackensen et al., 1993b;
McCorkle et al., 1997; Mackensen and Licari, 2004), bottom-water mass characteristics (Woodruff et al., 1980; Graham et al., 1981), and bottom-water circulation changes (Duplessy et al., 1984; Curry et al., 1988; Sarnthein et al., 1994; Bickert and Wefer, 1996; Mackensen et al., 2001; Bickert and Mackensen, 2004; Curry and Oppo, 2005). The truely epibenthic foraminiferal species Cibicidoides wuellerstorfi usually reflects the δ13C signal of the dissolved inorganic carbon (DIC) in the bottom water (Woodruff et al., 1980; Belanger et al., 1981; Graham et al., 1981; Duplessy et al., 1984; Zahn et al., 1986; Duplessy et al., 1988; McCorkle and Keigwin, 1994; Mackensen and Licari, 2004), whereas the δ13C signal of endobenthic species reflect the productivity linked pore-water δ13C gradient (Mackensen and Douglas, 1989; McCorkle et al., 1990; Loubere et al., 1995; Rathburn et al., 1996; McCorkle et al., 1997; Mackensen et al., 2000; Holsten et al., 2004; Mackensen and Licari, 2004; Schmiedl et al., 2004).
Reconstruction of bottom water δ13CDIC using δ13C values of C. wuellerstorfi was questioned, since productivity-linked lower δ13C values of this species were observed (Sarnthein et al., 1988; Mackensen et al., 1993b; Sarnthein et al., 1994; Bickert and Wefer, 1999). More recently, however, no productivity influence was recorded (Mackensen and Licari, 2004; Corliss et al., 2006; Eberwein and Mackensen, 2006). It was shown that δ13C values of all specimens of the same species vary little over the entire habitat depth in which they occur (Mackensen and Douglas, 1989; Rathburn et al., 1996; McCorkle et al., 1997; Mackensen et al., 2000; Mackensen and Licari, 2004; Schmiedl et al., 2004; Fontanier et al., 2006), which is important regarding paleoproductivity reconstructions. The approach of Δδ13C offsets (difference between δ13C values of epi- and endobenthic species) were used either to reconstruct (paleo-) productivity variations (Woodruff and Savin, 1985; Zahn et al., 1986; McCorkle et al., 1994;
Fontanier et al., 2006) or assumed to indicate bottom-water oxygen concentrations (McCorkle et al., 1997; Fontanier et al., 2006; Schmiedl and Mackensen, in press).
The influence of carbonate ion undersaturated bottom water masses on benthic foraminiferal isotopic compositions is only marginally investigated (Bemis et al., 1998; Mackensen and Licari, 2004).
Most recently, there is evidence that benthic foraminiferal 13C is extremly depleted in methane enriched settings (Wefer et al. 1994; Rathburn et al., 2000; Hill et al., 2003; Kennett et al., 2003; Rathburn et al., 2003; Mackensen et al., 2006). Intense methane releases are considered to have caused rapid climate changes. It seems promising to use δ13C values to detect methane flux variations and thus, to find out about their importance in climate changes. Summarizing I point out that benthic foraminifera provide a useful and reliable proxy in paleo-oceanographic reconstructions, although the involved and interacting biogeochemical processes need further investigations.
1.3 Aim of this thesis
This PhD thesis entitled ‘Holocene and Last Glacial Maximum (paleo-) productivity off Morocco: evidence from benthic foraminifera and stable carbon isotopes’ has been carried out within the subproject ‘Characterisation of high-productivity regions using microfossils and organic compounds’, which is part of the DFG Research Center Ocean Margins (RCOM).
The thesis has been divided into three main parts. The first part gives an introduction, which concentrates on the study area, its present and past climatic conditions as well as on a brief review of benthic foraminiferal research. The second and major part, which is composed of three manuscripts, focuses on the presentation and discussion of the results of the research conducted during this PhD thesis. The third part summarises the main conclusions of the second part and gives a perspective beyond outstanding problems.
1.3.1 Objectives
High-production systems, such as the NW-African coastal upwelling area are important regions concerning the organic carbon cycle. Particularly, glacial / interglacial variations in marine productivity are considered to determine atmospheric CO2 concentrations. The primary objective of this PhD thesis is to assign and reconstruct (paleo-) productivity conditions with the aid of benthic foraminiferal species and their stable carbon isotopic composition in a geographically limited area within the upwelling region off Morocco (NW-Africa). The central questions arising from this background include the following aspects:
• Does the Recent benthic foraminiferal community reflect present productivity conditions?
• To what extent is the isotopic composition of Recent benthic foraminifera affected by productivity?
• Can benthic foraminifera be used as reliable proxy to reconstruct paleoproductivity and distribution of past bottom water masses?
• Did paleoproductivity during the Last Glacial Maximum change as compared to productivity in the Holocene?
In order to answer these questions three manuscripts have been prepared, all of which have been submitted to international journals. The main topics of these manuscripts are summarised in the following.
1.3.2 Overview of Research
Eberwein, A. and Mackensen, A. (2006). Regional primary productivity differences off Morocco (NW- Africa) recorded by modern benthic foraminifera and their stable carbon isotopic composition. (Deep- Sea Research I).
We examine the correlation of live and dead benthic foraminifera with different environmental parameters, such as chlorophyll-a concentrations in surface waters (indicative of primary productivity) as inferred from satellite imagery, bottom water mass characteristica and sediment type. The discussion focuses on the effect of primary productivity on the composition and distribution of benthic foraminiferal faunas as well as the stable carbon isotopic composition of the most common species. Therefore, this study was conducted with a high spatial resolution to obtain a detailed picture of the modern primary productivity conditions. We further discuss the factors determining the similarities and discrepancies between live and dead benthic foraminiferal assemblages, to assign fundamental ecological information, which is a prerequisite for paleoproductivity reconstruction.
Eberwein, A. Mackensen, A.and Davenport, R. (submitted, in revision). The influence of organic matter fluxes on the microhabitat distribution of live benthic foraminifera off Cape Ghir and Cape Yubi (NW- Africa). (Journal of Foraminiferal Research).
In order to assess the impact of organic matter fluxes on the benthic foraminiferal community and the microhabitat distribution seven sediment surface samples, four of them located under the Cape Ghir filament, three of them under the Cape Yubi filament, were investigated for their live benthic foraminiferal content. Furthermore, we discuss the interplay between the quantity of exported organic matter and bottom water oxygen concentration in relation to the vertical zonation of benthic foraminifera.
In this context a special focus is set on the density and average living depth of the deep infaunal species Globobulimina affinis, since this species is considered to be tightly linked to the zone of zero oxygen concentrations.
Eberwein, A. and Mackensen, A. (submitted). Last Glacial Maximum productivity and water masses off NW-Africa: evidence from benthic foraminifera and stable isotopes. (Marine Micropaleontology).
We reconstruct paleoproductivity and stratification of water masses during the Last Glacial Maximum (LGM). The δ13C of Cibicidoides wuellerstorfi and equilibrium adjusted δ18O values of several species were used to determine the water mass stratification. Paleoproductivity was reconstructed with the aid of distinct assemblages and indicator species as well as the differences of δ13C values between epi- and infaunal species. Based on these proxies the main emphasis lies on the reconstruction of paleoproductivity and its comparison with Holocene productivity patterns.
2.1 Manuscript 1
Regional primary productivity differences off Morocco (NW-Africa) recorded by modern benthic foraminifera and their stable carbon isotopic composition
Eberwein, A. & Mackensen, A.
Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, D-27568 Bremerhaven, Germany
Keywords: Benthic foraminifera; Chlorophyll-a; Stable carbon isotopes; Organic matter; NW-Africa
Abstract
The influence of different primary productivity regimes on live (Rose Bengal stained) and dead benthic foraminiferal distribution, as well as on the stable carbon isotopic composition of foraminiferal tests, was investigated in sediment surface samples ((0-1 cm)) from the upwelling region off Morocco between Cape Ghir (31°N) and Cape Yubi (27°N). A combination of factor analysis, detrended correspondence analysis (DCA) and canonical correspondence analysis (CCA) was applied to the benthic foraminiferal data sets. Five major assemblages for both the live and dead fauna were revealed by factor analysis.
In the cape regions organic matter fluxes are enhanced by high chlorophyll-α concentrations in the overlying surface waters. Here, benthic foraminiferal faunas are characterized by identical live and dead assemblages, high standing stocks, and low species δ13C values, indicating constant year-round high productivity. Bulimina marginata dominates the unique fauna at the shallowest station off Cape Ghir indicating highest chlorophyll-a concentrations. Off both capes, the succession of the Bulimina aculeata / Uvigerina mediterranea assemblage, the Sphaeroidina bulloides / Gavelinopsis translucens assemblage, and the Hoeglundina elegans assemblage from the shelf to the deep sea reflects the decrease in chlorophyll-a concentrations, hence the export flux. In contrast, the area between the capes is characterized by differently composed live and dead assemblages, low standing stocks, and less depleted δ13C values, thus reflecting low primary productivity. High foraminiferal numbers of Epistominella exigua, Eponides pusillus, and Globocassidulina subglobosa in the dead fauna indicate a seasonally varying primary productivity signal. Significantly lower mean δ13C values were recorded in Bulimina mexicana, Cibicidoides kullenbergi, H. elegans, U. mediterranea and Uvigerina peregrina. Cibicidoides wuellerstorfi is a faithful recorder of bottom water δ13C in the Canary Islands regions. The mean δ13C signal of this species is not significantly influenced by constant high organic matter fluxes. The species- specific offset between live and dead specimens is the same.
1. Introduction
Benthic foraminifera are an important proxy in reconstructing primary productivity changes and corresponding organic matter fluxes to the sea floor (Altenbach and Sarnthein, 1989; Loubere, 1991;
Berger and Herguera, 1992; Linke and Lutze, 1993; De Rijk et al., 2000). The variation in primary productivity is responsible for the amount and quality of organic matter reaching the seafloor, which in turn influences the composition and standing stocks of benthic foraminiferal faunas (Lutze and Coulbourn, 1984; Caralp, 1989; Corliss and Emerson, 1990; Hermelin and Shimmield, 1990; Herguera and Berger, 1991; Schmiedl et al., 2000; Fontanier et al., 2002). The vertical distribution of living benthic foraminifera within the sediment is controlled mainly by the combination of organic matter flux and the oxygen concentration in bottom and pore waters (e.g. Corliss and Emerson, 1990; Jorissen et al., 1995).
Species also react with a microhabitat change in response to variations in these conditions (Jorissen et al., 1995). The importance of infaunal species increases significantly in Corg enriched sediments (Sen Gupta and Machain-Castillo, 1993) and in areas with high organic matter fluxes (De Rijk et al., 2000;
Gooday, 2003). In oligotrophic settings, where the labile fraction of the organic matter is consumed almost at the sediment-water interface, the fauna is characterized by epifaunal taxa (Linke and Lutze, 1993; Jorissen et al., 1995).
Various factors, including transport, dissolution and population dynamics, can influence the dead fauna (Murray, 1991; Loubere et al., 1993; Mackensen et al., 1995; Jorissen and Wittling, 1999), creating fundamental differences between the dead and live fauna at the same site. The live fauna represents a snapshot of actual environmental conditions, while the dead fauna represents a longer averaged time period. Therefore, knowledge about the differences between live and dead fauna is important to reconstruct paleoenvironmental conditions from the fossil fauna.
The stable carbon isotopic composition of benthic foraminiferal tests provides information about the carbon cycling in the world ocean and is widely used to reconstruct past deep-water circulation changes (Curry et al., 1988; Duplessy et al., 1988; Boyle, 1992; Sarnthein et al., 1994; Mackensen et al., 2001) as well as organic matter flux variations (e.g. Zahn et al., 1986; Loubere, 1987; Bickert and Wefer, 1999). The δ13C distribution of dissolved inorganic carbon (DIC) in the oceans is dependent on the interaction of biological uptake at the sea surface, air-sea gas exchange and organic matter degradation.
The benthic foraminiferal species Cibicidoides wuellerstorfi usually calcifies in 1:1 relationship with the ambient bottom water DIC (Belanger et al., 1981; Duplessy et al., 1984; Duplessy et al., 1988;
McCorkle and Keigwin, 1994). However, in areas with pulsed organic matter inputs even δ13C signals of C. wuellerstorfi are depleted (Mackensen et al., 1993b). Infaunal species are considered to record the pore-water δ13C gradient (McCorkle et al., 1997; Mackensen and Licari, 2004).
In eutrophic regions, the benthic foraminiferal community is characterized by significant numbers of specimens living deeper within the sediment (Jorissen et al., 1995). Under these conditions the number of deeply infaunal specimens may be strongly underestimated when only the uppermost centimetre is investigated. Licari and Mackensen (2005) showed that in eutrophic settings the overall
environmental signal, as deduced from the benthic foraminiferal community in the uppermost surface sediment (0 -1 cm), does not significantly differ from the information gained from the benthic foraminiferal fauna of the upper 10 cm. In very eutrophic regions, benthic foraminifera are concentrated at the sediment surface and, therefore, the investigation of the uppermost centimetre gives an accurate picture of the total fauna. Eutrophic conditions are predominant off Cape Ghir and Cape Yubi. Here, faunal differences within the sediment, different standing stocks, and variations in the average living depth of species were observed (Eberwein, unpubl. data). Still, the general conclusion drawn from the surface fauna (0 - 1 cm) is meaningful in terms of differences in overlying productivity regimes, which is the main purpose of this study.
In this study we describe the community pattern of live and dead benthic foraminifera from 30 surface sediment samples (0 - 1 cm) as well as the stable isotopic composition of the most common species in the upwelling region off Morocco. The high spatial resolution allows a better understanding of the relation between faunal changes, standing stocks and primary productivity. In addition, we examined the effect of productivity on benthic δ13C values. This study aims to provide a modern analogue for the reconstruction of Late Pleistocene productivity changes in the investigation area.
2. Oceanographic setting
The area under investigation (Fig. 1) is located in the eastern part of the subtropical North Atlantic gyre off Morocco (NW Africa) at the continental margin between 27°N and 31°N. This gyre represents the connection of the Gulf Stream via the Azores Current (AC) and the Canary Current (CC) with the North Equatorial Current. The study area is part of the Eastern Boundary Current (EBC) regime, which belongs to one of the major eastern boundary upwelling systems of the world (Hagen, 2001; Carr, 2002).
In general, the EBC regions are characterized by high primary productivity (Fig. 1). Thus, they play an important role in biogeochemical cycles, especially with respect to the carbon export to the sea floor. Off Morocco, upwelling of North Atlantic Central Water (NACW) (Mittelstaedt, 1991) occurs over the shelf and shelf break (Hagen, 2001) (Fig. 1). The upwelled water masses have a maximum extent of 70 km (Mittelstaedt, 1991; Hernández-Guerra and Nykjaer, 1997; Hagen, 2001). The intensity and occurence of upwelling depends on the position of the Azores High, which is located in its northernmost position during the boreal summer. During summer the trade wind belt affects the African coast between 32°N and 20°N. North of 25°N upwelling is predominant in summer and early fall (Wooster et al., 1976;
Mittelstaedt, 1991; Van Camp et al., 1991; Nykjaer and van Camp, 1994). This is supported by a recent study of fluxes of upwelling-related microorganisms between Lanzarote Island and the African coast (Abrantes et al., 2002).
Striking features of the area are filaments, which are situated off Cape Ghir and Cape Yubi (Fig. 1), where the upwelled waters are transported several hundred kilometers offshore (Van Camp et al., 1991; Nykjaer and van Camp, 1994; Hernández-Guerra and Nykjaer, 1997; Hagen, 2001). The filaments off Cape Ghir and Cape Yubi exist throughout the year, with a strong upwelling signal in summer and fall (Hernández-Guerra and Nykjaer, 1997). They are generated by the interaction of the current system and the coastal cape morphology (Hagen et al., 1996; Stevens and Johnson, 2003), and they play an important role in transport of nutrients from the shelf to the deep sea.
The water column is characterized by the southward flowing CC down to 100 m depth (Knoll et al., 2002; Llinás et al., 2002). Below the CC the southward flowing NACW occurs down to a depth of 600 m, with increasing portions of the northwards directed South Atlantic Central Water (SACW) to the south (Knoll et al., 2002; Llinás et al., 2002). The southward directed Mediterranean Outflow Water (MOW) underlies the NACW down to a depth of 1700 m (Knoll et al., 2002; Llinás et al., 2002). The impact of the northwards flowing Antarctic Intermediate Water (AAIW) on the MOW characteristics Fig.1: Study area, chlorophyll-a concentration in surface waters, bathymetry and location of the 30 sediment sur- face samples (0 - 1 cm). SeaWifs derived chlorophyll-a concentrations (mean March 1998; Helmke, pers. comm) that represent > 1 mg m-3 are shaded dark grey, 0.4 – 1 mg m-3 light grey and < 0.4 mg m-3 not shaded. Surface currents (AC: Azore Current, CC: Canary Current) are indicated by arrows after Mittelsteadt (1991). Isobaths indicate 500, 1000, 1500 and 2000 m water depths. Investigated sediment surface samples are marked by squares.
Numbers represent stations, and transects are labled Cape Ghir, A, B, C, D and Cape Yubi.
Latitude
Longitude Canary Islands
30° 30°
32°
28° 28°
26° 26°
-12° -10°
-14°
-18° -16°
5546 5542 5541 5540 5539
42364237 42344235 42324233 4229423042314223
4228
4225 4226 4227 4217
42134212 4214 4215
4216 4207
6005 6006
6007 6008
AC
CC
CC
Cape Ghir
Cape Yubi
D C
B A -8°
-10°
Gulfstream
North Equatorial Current Subtropical
Gyre
increases to the south. The deepest watermass is the southward flowing North Atlantic Deep Water (NADW) (Knoll et al., 2002; Llinás et al., 2002).
The investigated sites are positioned in the transition zone between upwelled eutrophic waters and the oligotrophic waters of the subtropical North Atlantic gyre in order to detect the influence of trophic differences on the benthic foraminiferal community.
3. Material and methods
A total of 30 sediment surface samples was collected with a multiple corer and a giant box corer in the upwelling region off Morocco (NW-Africa) from water depths ranging between 355 m and 2504 m. The investigated sediment surface samples are distributed along six transects, which are located between 31°N and 27°N. In the following the transects will be labelled Cape Ghir, A, B, C, D, and Cape Yubi transect from north to south (Fig. 1). The sediment surface samples were recovered during cruises in 1996, 1998 and 1999 with RV Meteor (Table 1).
The sediment samples (0 – 1 cm) were stored in a Rose Bengal / 95 % ethanol mixture (1 g / 1 L) and kept at 4 °C. They were washed over 125-µm and 63-µm sieves and dried. The procedure of staining with Rose Bengal has limitations (e.g. Douglas et al., 1980; Bernhard, 1988; Corliss and Emerson, 1990). However, it is still the most common and practical method to quantify living foraminiferal faunas (Murray and Bowser, 2000). Benthic foraminifera were analyzed from the fraction > 125 µm and determined to species level. Samples were split into aliquots of at least 300 individuals (Phleger, 1960; Fatela and Taborda, 2002) for both the living and dead faunas. We counted proloculi seperately for branch-like agglutinated foraminifera and assumed three fragments to represent one individual in samples without proloculi. The standing stocks of living specimens and the foraminiferal numbers of dead specimens are expressed in numbers of individuals per 50 cm3 and were calculated for the fauna in the 0 – 1 cm interval and for the most abundant species.
A factor analysis with varimax rotation was applied in order to detect the composition of distinct assemblages and to reveal the important species of these assemblages, using the software SYSTATTM 5.2.1. Species that occured at least at two stations with a minimum abundance of 1 % were included in the analysis. Some commonly used statistical methods in micropaleontological studies are based on the assumption that the abundances of species show a linear response to environmental parameters. This assumption does not hold in many cases, because of the fact that species show a Gaussian (unimodal) distribution with respect to environmental parameters. Therefore, we used the two-step approach of a detrended correspondence analysis (DCA) and a canonical correspondence analysis (CCA) (Ter Braak, 1986) of the CANOCOTM software (Version 4.0).
Table 1: List of surface samples investigated in this study with sampling date, position, water depth, and coring device (MUC: Multiple Corer, GBC: Giant Box Corer).
Core Cruise Date Device Latitude Longitude Water depth
GeoB (°N) (°W) (m)
Transect Cape Ghir
6008-2 M45/5 Oct / Nov 1999 MUC 30°50,7 10°05,9 355 6007-1 M45/5 Oct / Nov 1999 MUC 30°51,1 10°16,0 899 6006-2 M45/5 Oct / Nov 1999 MUC 30°52,1 10°37,8 1275 6005-1 M45/5 Oct / Nov 1999 MUC 30°52,8 10°53,8 1781 4207-1 M37/1 Dec 1996 GBC 30°51,8 11°04,3 2123 Transect A
4212-3 M37/1 Dec 1996 MUC 29°36,2 10°57,0 1256 4213-1 M37/1 Dec 1996 MUC 29°41,8 11°04,7 1547 4214-3 M37/1 Dec 1996 MUC 29°46,9 11°11,8 1788 4215-1 M37/1 Dec 1996 MUC 30°02,2 11°33,2 2106 4216-2 M37/1 Dec 1996 MUC 30°37,9 12°23,8 2325 Transect B
4225-3 M37/1 Dec 1996 MUC 29°16,5 11°46,9 1281 4226-1 M37/1 Dec 1996 MUC 29°19,2 11°50,0 1400 4227-1 M37/1 Dec 1996 MUC 29°46,1 12°20,2 1826 4217-1 M37/1 Dec 1996 MUC 30°26,1 12°53,7 2504 Transect C
4223-1 M37/1 Dec 1996 MUC 29°01,1 12°28,0 777 4231-2 M37/1 Dec 1996 MUC 29°05,2 12°33,1 1197 4230-1 M37/1 Dec 1996 MUC 29°07,7 12°35,8 1316 4229-2 M37/1 Dec 1996 MUC 29°10,9 12°38,3 1422 4228-1 M37/1 Dec 1996 MUC 29°28,2 12°59,4 1633 Transect D
4237-1 M37/1 Dec 1996 MUC 28°43,7 13°01,0 800 4236-2 M37/1 Dec 1996 MUC 28°47,0 13°05,7 1030 4235-1 M37/1 Dec 1996 MUC 28°51,4 13°11,4 1247 4234-1 M37/1 Dec 1996 MUC 28°53,4 13°13,6 1360 4233-2 M37/1 Dec 1996 MUC 28°58,5 13°19,8 1303 4232-1 M37/1 Dec 1996 MUC 29°01,3 13°23,2 1161 Transect Cape Yubi
5546-3 M42/4 Oct 1998 MUC 27°32,2 13°44,2 1071 5542-3 M42/4 Oct 1998 MUC 27°32,2 13°50,8 1431 5541-2 M42/4 Oct 1998 MUC 27°32,2 13°59,7 1748 5540-3 M42/4 Oct 1998 MUC 27°32,1 14°10,5 2035 5539-2 M42/4 Oct 1998 MUC 27°32,2 14°21,3 2202
Table 2: Live and dead assemblages revealed by factor analysis with their dominant (score > 3) and associated (score > 1) species. Variance of each assemblage and total variance are given in percentages for both live and dead fauna.
Variance Dominant species Score Associated species Score (%)
Live fauna 21.18 Trifarina bradyi 7.68 Uvigerina mediterranea 2.96 Cibicidoides bradyi 2.47
Total variance Reophax scorpiurus 1.35
67.69% Epistominella rugosa 1.29
Gavelinopsis translucens 1.21 Cribrostomoides subglobosus 1.20 Melonis barleeanum 1.19 15.73 Hoeglundina elegans 9.09 Cribrostomoides subglobosus 1.43 Cibicidoides kullenbergi 1.43 Karrerulina conversa 1.03
Bulimina rostrata 1.02
11.85 Bulimina aculeata 7.03 Hyalinea baltica 2.46 Uvigerina mediterranea 5.09 Gavelinopsis translucens 1.70 11.39 Gavelinopsis translucens 7.38 Cibicidoides kullenbergi 2.81 Sphaeroidina bulloides 3.63 Bulimina mexicana 1.74 Cibicidoides bradyi 1.04 7.54 Cribrostomoides jeffreysii 5.18 Reophax scorpiurus 2.89 Cibicidoides kullenbergi 4.77 Reophax bilocularis 2.53 Cribrostomoides subglobosus 1.65
Eponides pusillus 1.59
Eggerella bradyi 1.32
Uvigerina mediterranea 1.24 Dead fauna 21.41 Epistominella rugosa 4.69 Cassidulina obtusa 2.84 Pyrgoella irregularis 4.45 Globocassidulina subglobosa 2.16
Total variance Uvigerina proboscidea 1.94
68.53% Trifarina bradyi 1.74
Miliolinella subrotunda 1.20 Gavelinopsis translucens 1.19 Lobatula lobatulus 1.15 Cibicidoides bradyi 1.11 Lagenammina difflugiformis 1.08 Karreriella bradyi 1.06 9.88 Bulimina aculeata -6.77 Epistominella rugosa -2.72
Uvigerina mediterranea -4.65 Hyalinea baltica -1.86 Melonis barleeanum -1.30 Uvigerina peregrina -1.16 10.40 Hoeglundina elegans 7.71 Cibicidoides kullenbergi 2.78
Bulimina mexicana 2.41
Bulimina rostrata 1.92
Cibicidoides wuellerstorfi 1.57 17.14 Gavelinopsis translucens 8.53 Bulimina mexicana 2.33 Cassidulina laevigata 1.41 9.70 Pyrgoella irregularis 4.54 Cribrostomoides jeffreysi 2.68 Cibicidoides wuellerstorfi 2.24
Eponides pusillus 2.10
Epistominella exigua 1.87 Cibicidoides kullenbergi 1.85 Cribrostomoides wiesneri 1.47 Ammodiscus incertus 1.38 Paratrochammina challengeri 1.18 Cribrostomoides subglobosus 1.17 Reophax scorpiurus 1.02
The basic assumption of DCA is that the main species and site variations are caused by one environmental variable or a combination of environmental parameters. The direction of the main variation of species and sites is represented by the first DCA axis, whereas higher axes are related to other environmental parameters, which show decreasing importance for the variation. A species rises to its mode and disappears again within four standard deviations (SD). Consequently, samples that plot more than four SD’s apart have no species in common (Hill and Gauch, 1980). If the length of the first axis is greater than 2 SD, unimodal response model can be assumed. In CCA, the species abundances are directly related to the environmental variables. The species data were transformed to approximately normal distributions, which is a prerequisite for CCA, by taking the logarithms. The environmental data set was standardized to have a mean equal to zero and a standard deviation equal to one. The Monte Carlo method with 200 permutations was used to test the significance of single environmental variables. The graphical representation of species and sites is given by points, whereas environmental variables are represented by arrows. The perpendicular projection of a species on the environmental parameter indicates the importance of this variable for the variation within this species. We included the relative abundance of the most important species revealed by factor analysis (Table 2) in the DCA / CCA analysis.
The environmental parameter data set (Table 3) for the CCA was taken from various published sources. Surface-water chlorophyll-α concentrations, sedimentary TOC contents, number of fragmented planktic foraminifera and carbonate contents are from Meggers et al. (2002). Silt and sand proportions are from Holz et al. (2004). Data from Meggers et al. (2002) and Holz et al. (2004) are available at www.pangaea.de. Salinity, bottom-water velocities and O2 concentrations are from Knoll et al. (2002), Llinás et al. (2002), Hernández-Guerra et al. (2003), Sarnthein et al. (1982), and Wefer et al. (1999). All environmental parameters were obtained from the same stations as the foraminiferal samples.
Bottom water samples were taken from directly above the sediment-water interface for the determination of the stable carbon isotopic composition of bottom water dissolved inorganic carbon (DIC). Samples were poisoned with HgCl2, sealed with wax, and kept at 4°C. The DIC of bottom water was extracted with a Finnigan Gas Bench using gas chromatography to purify CO2. The stable carbon isotopic composition of the resulting CO2 was determined with a Finnigan MAT 252 isotope ratio gas mass spectrometer and calibrated to VPDB (Vienna Pee Dee Belemnite). All water samples were run in duplicate. The overall precision is better than ± 0.1 ‰. The stable carbon isotopic composition of live and dead individuals of Bulimina aculeata (3-13 / 6-13, i.e. 3 to 13 live specimens and 6 to 13 dead specimens were measured), Bulimina mexicana (3-12 / 2-18), Cibicidoides kullenbergi (2-4 / 2-6), Cibicidoides wuellerstorfi (1-4 / 1-6), Hoeglundina elegans (2-4 / 1-4), Uvigerina mediterranea (1-11 / 2-9), and Uvigerina peregrina (3-9 / 3-8) was determined with a Finnigan MAT 251 isotope ratio gas mass spectrometer. The foraminiferal isotopic values are reported in δ–notation relative to the VPDB- scale with a precision better than ± 0.06 ‰ for δ13C.
Table 3: The environmental data used in the statistical analysis is listed. Surface water chlorophyll-a concentrations, sedimentary TOC contents, number of fragmented planktic foraminifera and carbonate contents are from Meggers et al. (2002). Salinity, bottom water velocities and O2 concentrations are from Knoll et al. (2002), Llinás et al. (2002), Hernández-Guerra et al. (2003), Sarnthein et al. (1982), and Wefer et al. (1999). Silt and sand proportions are from Holz et al. (2004). Addtionally, water depth, water mass and the dominant species of the live and dead assemblages are given. Data from Meggers et al. (2002) and Holz et al. (2004) are available at www. pangaea.de. GeoBWaterdepthWatermassDominant speciesDominant speciesDistance fromTOCChlorophyllSiltSandCarbonateFragmentedO2SalinityV station(m)of live faunaof dead faunaAfrican coast(%)Ann. mean (1998)(wt-%)(wt-%)(%)plank.foram.ml/l(Sv) (km)(mg/cm3)(%) Transect Cape Ghir 6008355NACWB. marginataB. spathulata221.652.2742.360.4624.9124.684.5235.750.80 6007899MOWB. aculeata / U. mediterraneaB. aculeata / U. mediterranea421.871.4162.531.5825.3530.836.2435.600.10 60061275MOWB. aculeata / U. mediterraneaG. translucens761.120.8149.409.9030.7632.336.8935.600.05 60051781NADWG. translucens / S. bulloidesG. translucens1021.060.3844.345.9433.3723.467.8534.900.90 42072123NADWH. elegansH. elegans1171.040.3735.4720.545.3034.900.90 Transect A 42121256MOWT. bradyiE. rugosa890.730.2855.8822.5654.4513.703.9035.600.05 42131547NADWH. elegansE. rugosa1100.740.2831.6337.0761.077.805.3034.900.05 42141788NADWG. translucens / S. bulloidesP. irregularis1270.800.2766.8716.105.3034.900.90 42152106NADWC. jeffreysi / C. kullenbergiP. irregularis1750.780.2425.0451.2667.6512.605.3034.900.90 42162325NADWC. jeffreysi / C. kullenbergiP. irregularis2700.590.2031.0957.2971.698.485.3034.900.90 Transect B 42251281MOWT. bradyiG. translucens1470.730.2537.7727.5955.5010.143.9035.600.05 42261400MOWT. bradyiE. rugosa1590.740.1934.0832.7159.759.623.9035.600.05 42271826NADWC. jeffreysi / C. kullenbergiP. irregularis2340.800.1964.134.355.3034.900.90 42172504NADWH. elegansP. irregularis3160.540.2069.154.005.3034.900.90 Transect C 4223777MOWT. bradyiB. aculeata / U. mediterranea1860.740.2451.3812.5952.6913.873.9035.600.10 42311197MOWT. bradyiP. irregularis2030.850.2144.0323.4555.6810.283.9035.600.05 42301316MOWT. bradyiP. irregularis2080.770.2141.1820.5855.985.693.9035.600.05 42291422MOWT. bradyiG. translucens2170.820.2047.3626.4757.6610.743.9035.600.05 42281633NADWT. bradyiP. irregularis2770.690.1938.7138.0862.636.285.3034.900.90 Transect D 4237800MOWB. aculeata / U. mediterraneaB. aculeata / U. mediterranea1830.930.2756.3915.1648.6019.623.9035.600.10 42361030MOWT. bradyiG. translucens1950.790.2744.3219.2548.3918.093.9035.600.05 42351247MOWT. bradyiG. translucens2220.910.2544.4817.2749.4113.213.9035.600.05 42341360MOWT. bradyiE. rugosa2340.810.2648.496.353.9035.600.05 42331303MOWT. bradyiG. translucens2570.810.2546.1325.3047.879.733.9035.600.05 42321161MOWT. bradyiP. irregularis2690.780.2540.1535.5149.8413.533.9035.600.05 Transect Cape Yubi 55461071AAIWB. aculeata / U. mediterraneaB. aculeata / U. mediterranea491.550.5063.026.2435.6032.013.8035.310.05 55421431AAIWG. translucens / S. bulloidesG. translucens571.250.5045.508.1935.1229.084.7035.300.05 55411748NADWG. translucens / S. bulloidesG. translucens731.220.4049.136.9636.8625.005.1035.180.90 55402035NADWH. elegansH. elegans901.140.3044.337.8536.7428.395.2035.100.90 55392202NADWH. elegansH. elegans1071.030.2844.4111.4438.6115.995.4034.900.90
4. Results
4.1 Live and dead faunas
A total of 243 live and 281 dead species was identified. The foraminiferal standing stocks of surface samples (0 – 1 cm) vary between 24 and 3311 individuals per 50 cm3 (Table 4). Generally, standing stocks are higher in the cape regions than in the area between the capes. Highest standing stocks are found at stations GeoB 6008-2 and GeoB 6007-1 off Cape Ghir. Furthermore, standing stocks decrease with increasing water depth on each transect (Table 4). Foraminiferal numbers vary between 267 and 18960 individuals per 50 cm3 (Table 5). They do not show a decrease with increasing water depth as observed for standing stock values.
The factor analysis revealed five factors consisting of 21 important species for the living fauna (Table 2), and five factors consisting of 31 important species for the dead fauna (Table 2). The surface fauna at station GeoB 6008-2 shows a unique faunal composition in comparison to all other faunal assemblages (Figs. 2, 3). It is a striking observation that live and dead assemblages in the sediment surface show an almost identical faunal composition and succession on the Cape Ghir and Cape Yubi transect, while they differ in their composition and distribution in the area between the capes (Figs. 2, 3).
In DCA of the live and dead fauna, the lengths of the first and second axes exceed two standard deviations (Table 6), indicating that the species show a unimodal distribution in response to environmental parameters. The grouping of assemblages obtained from factor analysis is confirmed by DCA analysis (Figs. 4, 5). For the live fauna the first axis most likely represents a combination of water depth, chlorophyll-α concentration in surface waters and the sand content of the sediment. The second axis represents a combination of salinity, fragmented planktic foraminiferal content in the sediment and chlorophyll-α concentration in surface waters (Fig. 6, Table 6). For the dead fauna the significant parameters differ slightly. The first axis most likely represents a combination of water depth, carbonate content in the sediment, chlorophyll-α concentration in surface waters and distance from the African coast. The second axis represents a combination of salinity and chlorophyll-α concentration in surface waters (Fig. 7, Table 6).
A unique live fauna at the shallowest station GeoB 6008-2 (355 m) is characterized by Bulimina marginata, Brizalina spathulata, Bigeneria cylindrica, and Uvigerina elongatastriata (Fig. 2, Table 4). The dead fauna is characterized by Brizalina spathulata, Gyroidina umbonata, B. marginata, Chilostomella oolina, Cassidulina laevigata, and Bulimina striata (Fig. 3, Table 5). At station GeoB 6008-2 live and dead faunas show affinities to the highest chlorophyll-α concentration (2.3 mg / m3) and salinity (35.75) (Figs. 6, 7, Table 3). The live fauna can also be attributed to an intermediate fragmented planktic foraminiferal content (25 %) and a very low sand content (0.5 %) (Fig. 6, Table 3). The dead fauna occurs at this station with intermediate carbonate content (25 %) and proximity to the African coast (22 km) (Fig. 7, Table 3).
Table 4: Standing stock values in individuals per 50 cm3 of surface fauna (0 – 1 cm) of dominant, associated, and remaining species as well as water depths.
Table 5: Foraminiferal numbers of empty tests in individuals per 50 cm3 of surface fauna (0 – 1 cm) of dominant, associated, and remaining species as well as water
CapeGhir
CapeYubi
Bulimina marginata Bulimina aculeata / Uvigerina mediterranea Gavelinopsis translucens / Sphaeroidina bulloides Hoeglundina elegans Trifarina bradyi
Cribrostomoides jeffreysii / Cibicidoides kullenbergi
2000 m
1500 m 1000 m 500 m
D C
B A
Longitude
-16° -15° -14° -13° -12° -11° -10° -9°
Latitude 30°
31°
32°
29°
27°
28°
Fig. 2: Spatial composition and distribution of the five live assemblages in sediment surface samples and their do- minant species revealed by factor analysis. Station GeoB 6008-2 (square) does not fit in any assemblage because of its unique faunal composition. Cape Ghir, A, B, C, D and Cape Yubi represent transects.
CapeGhir
CapeYubi
Brizalina spathulata Bulimina aculeata / Uvigerina mediterranea Gavelinopsis translucens Hoeglundina elegans Epistominella rugosa / Pyrgoella irregularis Pyrgoella irregularis
D C
B A
2000 m
1500 m 1000 m 500 m
Longitude
-16° -15° -14° -13° -12° -11° -10° -9°
Latitude
30°
31°
32°
29°
27°
28°
Fig. 3: Spatial composition and distribution of the five dead assemblages in sediment surface samples and their dominant species revealed by factor analysis. Station GeoB 6008-2 (square) does not fit in any assemblage because of its unique faunal composition. Cape Ghir, A, B, C; D and Cape Yubi represent transects.
Table 6: Results of the detrended correspondence analysis (DCA), and the canonical correspondence analysis (CCA) of the live and dead faunal data set. The length of the first and second axis in standard deviations (SD) and the variance (in %) explained by the axis of the DCA are listed. The significant environmental variables (95%, p value < 0.05, bold letters) are listed with their correlation coefficient for the first and second axis. Lambda (mul- tiplied by 100) indicates the significance of the environmental parameter in percent.
DCA axis 1 DCA axis 2 CCA
SD Variance [%] SD Variance [%] Variable lambda p value Axis1 Axis2
Live 4.04 58.9 2.04 21.2 Water depth 0.22 0.01 -0.83 0.42
Chlorophyll 0.11 0.01 0.64 0.55
Fragmented PF 0.04 0.01 0.50 0.58
Salinity 0.03 0.03 0.48 -0.62
Sand 0.02 0.03 -0.52 -0.24
Silt 0.03 0.08 0.67 0.08
Velocity 0.03 0.14 -0.60 0.60
O2 0.02 0.27 -0.55 0.63
Carbonate 0.01 0.60 -0.52 -0.38
TOC 0.01 0.44 0.57 0.57
Distance f. African coast 0.01 1.00 -0.39 -0.50
Dead 4.16 61.3 2.75 25.3 Water depth 0.21 0.01 -0.82 0.43
Carbonate 0.14 0.01 -0.76 -0.42
Chlorophyll 0.10 0.01 0.67 0.51
Salinity 0.04 0.01 0.53 -0.71
Distance f. African coast0.02 0.03 -0.52 -0.48 Fragmented PF 0.02 0.10 0.64 0.53
Velocity 0.02 0.17 -0.57 0.67
Silt 0.01 0.38 0.64 0.11
O2 0.02 0.57 -0.54 0.68
TOC 0.01 0.65 0.69 0.59
Sand 0.01 0.65 -0.64 -0.34
